The immune system protects the body from infectious agents and disease and is critical to our survival. However, in certain instances, the immune system can be the cause of illness. One example is in autoimmune disease wherein the immune system attacks its own host tissues, in many instances causing debilitating illness and sometimes resulting in death. Examples of autoimmune diseases include multiple sclerosis, type 1 insulin-dependent diabetes mellitus, lupus erythematosus and arthritis. A second example where the immune system can cause illness is during tissue or organ transplantation. Except in the cases of genetically identical animals, such as monozygotic twins, tissue and organ transplants are rejected by the recipient's immune system as foreign. The immune reaction against transplants is even more pronounced in transplantation across species or xenotransplantation. A third example where the immune system harms the host is during an allergic reaction where the immune system is activated by a generally innocuous antigen causing inflammation and in some cases tissue damage. A fourth example where the immune system is involved is in fetal loss.
In order to inhibit the detrimental immune reactions during transplantation, autoimmune disease and allergic reactions, immunosuppressive drugs (such as cyclosporin A, tacrolimas, and corticosteroids) or antibody therapies (such as anti-T cell antibodies) are generally administered. Unfortunately, these non-specific modes of immunosuppression generally have undesirable side effects. For example, cyclosporin may cause decreased renal function, hypertension, toxicity and it must be administered for the life of the patient. Corticosteroids may cause decreased resistance to infection, painful arthritis, osteoporosis and cataracts. The anti-T cell antibodies may cause fever, hypertension, diarrhea or sterile meningitis and are quite expensive.
In view of the problems associated with immunosuppression, there has been an interest in developing methods or therapies that induce unresponsiveness or tolerance in the host to a transplant, to “self” tissues in autoimmune disease and to harmless antigens associated with allergies. The inventors have been studying the mechanisms involved in transplant rejection and have developed methods for inducing a state of antigen-specific immunological tolerance in transplantation. In particular, in animal allograft models, the inventors have demonstrated that graft survival, such as renal and skin allograft survival, can be increased if the recipient animal is given a pre-transplant infusion via the portal vein of irradiated spleen cells from the donor animal (2,3). In contrast, a pre-transplant infusion via the tail vein does not prolong graft survival. Increased graft survival was further shown to be in turn associated with increased expression of a number of distinct mRNAs (4), one of which encodes CD200 (previously called OX2), a molecule expressed on the surface of dendritic cells (5).
The CD200 protein (also known as OX2) has a high degree of homology to molecules of the immunoglobulin gene family, which includes molecules important in lymphocyte antigen recognition and cell-cell interaction (e.g. CD4, CD8, ICAMs, VCAMs), as well as adhesion receptor molecules (NCAMs) in the nervous system. However, prior to the present inventors, the function of the CD200 protein was largely unknown. The present inventors showed subsequently that infusion of anti-CD200 monoclonal antibodies from the time of transplantation blocks the protective effect of pv immunization in mice receiving renal allografts (4) and rats receiving SIT (6), and the polarization to type-2 cytokine production seen in these models (4, 6). A soluble immunoadhesin, in which the extracellular domain of CD200 was linked to a murine IgG2aFc region, inhibited T-cell allostimulation and type-1 cytokine production (IL-2, IFNγ) in vitro and in vivo (1). Since the intracellular domain of CD200 lacks signalling motifs, or any docking sites for adapter molecules which might engage an intracellular signalling cascade, the present inventors suggest that these and other data (4,7,25) are consistent with the idea that engagement of a receptor for CD200 (CD200R) by CD200 may deliver key immunoregulatory signals (36).
T-cells are activated after concomitant engagement of TCRs with antigen presented on APC in association with MHC molecules and the delivery of costimulatory signals resulting from the interaction of several ligand:coreceptor complexes (8-11). Major positive costimulatory interactions include the following: CD40L with CD40, and CD28 with CD80/CD86; CTLA4 interactions with CD80/CD86 may deliver a negative signal (12-17). While positive costimulatory signals are clearly important in T-cell triggering, blocking this costimulation alone, and/or facilitating signalling via CTLA4, has not reproducibly induced tolerance. This may reflect the need for other molecules (such as CD200) in active immunoregulation (4). In recent studies the inventors reported that dendritic cells (DC) expressing CD200, triggered an immunoregulatory function leading to increased allograft survival. Moreover, these cells were physically distinguishable from those DC with optimal allostimulatory capacity (7).
Early attempts to characterize CD200R by Preston et al. (18) were performed by constructing a soluble chimeric protein with the extracellular domains of CD200 engineered onto domains 3+4 of rat CD4 antigen. In order to detect weak interactions, the chimeric protein was coupled to fluorescent covaspheres to ensure an avid display of CD200. These CD200 covaspheres were reported to bind to macrophages but not other cell types. The specificity of the interaction was documented by inhibition studies using Fab fragments of the CD200 monoclonal antibody (mAb). Using site-directed mutagenesis this group further reported results suggesting that the ligand-binding domain of CD200 was in the NH2-terminal domain of the extracellular region of CD200.
Recently, Barclay et al. reported several forms of the CD200R (WO 00/70045, published Nov. 23, 2000).